In the last twenty years, the world has seen a revolution in communications. Twenty years ago, landline telephones were the dominating means for communications. Since then, GSM (Global System for Mobile communication) and its different equivalents or counterparts have become widely used all over the world and also EDGE (Enhanced Data rates for Global Evolution) and UMTS (Universal Mobile Telecommunications System) also known as 3G (Third Generation). Another development is that data communications are merging more and more with mobile telephone communications. These days, surfing the Internet, sending short messages and e-mails using a mobile phone, PDA (Personal Digital Assistant), laptop or the like, away from home and a landline connection, are as natural as making a landline phone call was some twenty or thirty years ago. The technical evolution has been fast and is still ongoing.
A requirement for allowing extensive use of mobile stations, such as a mobile phone, PDA, laptop or the like, for surfing the Internet and using various new services that are constantly evolving, is a high bit rate in the radio interface. A requirement for new services to evolve and for the existing relatively new services to become successful and widely used, is that the bit rate in the radio interface can be increased compared to the communication systems existing or in use today.
A new technique for increasing the bit rate in radio communication is LTE, Long Term Evolution. This technique can be used to provide bit rates around 100 Mbit/s, and perhaps even higher. Working together with LTE is the IP Multimedia Subsystems, IMS. IMS is an architectural framework for delivering Internet Protocol (IP) multimedia services. To ease the integration with the Internet, IMS uses IETF protocols wherever possible, e.g. the Session Initiation Protocol (SIP).
As the different new techniques and services are being developed, it is important that they are also compatible with “old” existing techniques and also that existing services will be supported by the new techniques. For example, today GSM and UMTS systems are compatible and are interworking solutions for communication and a GSM user can interact with a UMTS user without even knowing if the other is using GSM or UMTS.
An example of an old existing service that is to be supported by LTE is SMS (Short Messaging Service). In 3GPP23.204, it is described that all existing SMS capabilities are supported through SMS encapsulation in IP.
In IMS, a user may be reachable on several different user equipments (formerly referred to as mobile stations in GSM and UMTS). In IMS, two different identities are used, IMPI and IMPU. The IMPI (IP Multimedia Private Identity) is unique to a user equipment or subscription and not necessarily to a specific user. The IMPU (IP Multimedia Public Identity) can be shared in more than one user equipment, so that any of them can be reached with the same identity. For example, a single phone-number may be valid for an entire family using plural user equipments. The IMPI and the IMPU are both URIs, for example a tel-uri or a sip-uri. A URI is a Uniform Resource Identifier. A tel-uri may comprise a telephone number and a sip-uri may e.g. comprise an e-mail address. An IMPU is unique to a user, or a user group, having an IMS profile.
The private identity, IMPI, is used, e.g., for authentication during registration, re-registration and de-registration, and it is therefore also stored in an HSS (Home Subscriber Server) node.
The public identity, IMPU, can be used by any services/users to identify a subscriber and/or his/her equipments when participating in IMS sessions.
The problem that arises with existing solutions is that a terminating SIP message is destined to a particular IMS user and not necessarily to a particular device, in case the user has multiple user equipments under the same subscription. For normal chat messages, it may very well be the case that the user would like to have the messages forwarded to all user equipments. But for, e.g., configuration messages, it may even be harmful to forward such messages to user equipments that are not supposed to receive them.
Moreover, the use of SIP implies that a terminating message is received by the SIP application and not by the CS (Circuit Switched) application on the user equipment. Hence in any of the above cases, it is not clear how to ensure that the intended user equipment receives the message, as well as the correct application as such in the terminal, which message may be e.g. an OTA (Over The Air) configuration message.
The solution in IMS today that is used to direct messages to a particular user equipment, is the use of a parameter called GRUU (Globally Routable UA URI). This is not always possible to use, as the notion of GRUU does not exist in the CS world. As a consequence, GRUU would only be possible to employ for networks where IMS is used end-to-end, and it would not be possible to “breakout” to a user equipment that is registered in a CS network. Here, to breakout means to (via a gateway) forward a message to the CS domain.